64 
N. Bancroft. 
xylem which appears later in these Stangeria bundles is held by 
Worsdell to be an ancestral character. He regards it as being 
usually the homologue of the central part of such a stele as that of 
Medullosa anglicn , though it may correspond to the oppositely 
orientated xylem of the original strand. Scott, as we have seen, 
believes that the centripetal xylem, as found in the peduncle of 
Stangeria and other Cycads is the homologue and derivative of the 
centripetal primary xylem (if it be centripetal!) occurring in the 
bundles of the vegetative stems of Lyginopteris and Heterangium. 
In this, of course, Worsdell and Scott agree, but they differ as to 
the mode of origin of the vascular tissue in these fossil plants. 
According to Scott, the centripetal xylem of Lyginopteris is all that 
remains of the mass of primary xylem, which, with conjunctive 
parenchyma, occupies the centre of the Heterangium stele. Worsdell, 
on the contrary, holds that in Lyginopteris each primary xylem 
group with its accompanying secondary centrifugal xylem and 
phloem is “ the one-sided remnant of an entire stele or concentric 
bundle, and is hence precisely comparable to, and homologous with, 
each bundle in the peduncle of Stangeria ” and other forms; in 
both cases, the ancestral form of vascular structure would be a 
stele like one of those of a Medullosean stem. His reasons for so 
believing depend largely upon the sinuous contour of the secondary 
xylem in Lyginopteris indicating its compounded character (Fig. 
18, ns); and upon the formation of concentric strands of vascular 
tissue in place of the ordinary bundles of the stem, by the growth 
of the cambium through the leaf-gaps (Fig. 18, b), which Worsdell 
Fig. 18 .—Lyginopteris oldhamia. A, transverse section of vascular ring of 
stem of young plant. (Adapted from Worsdell, 1906, after Williamson and 
Scott). 
B, transverse section of single bundle from vascular ring of stem, showing 
its concentric stele lilie structure, caused by extension of the cambium through 
leaf-gaps. (After Worsdell, 1906). 
regards as a reversion to an ancestral character. So that, according 
to him, not only is the Cycadean vascular system traceable to an 
origin amongst polystelic types, but that of Lyginopteris also. 
The fertile portions of the axis of the male cone in Ceratozamia 
mexicana and Ceratozamia latifolia exhibit the rudiments of an 
